This invention relates generally to data transmission cables and more specifically to a high speed data transmission cable which has multiple primary cable pairs combined together into a larger cable structure.
There is currently a demand for high speed data transmission cables which are capable of high-fidelity data signal transmission at minimal signal attenuation. The ever-increasing use of high speed computer equipment and telecommunications equipment has increased such demand.
One existing cable product capable of high data rate transmission is fiber-optic cable which has good bandwidth performance over long distances. Furthermore, fiber-optic cables provide very low attenuation and little interference or noise with the transmitted signal. However, despite their desirable signal transmission qualities, fiber-optic cables are still very expensive. Furthermore, when transmission of signals over shorter distances is required, fiber-optic cables become particularly less desirable from an economic standpoint. As a result, for high speed data transmission over relatively short distances, such as up to 50 meters, copper based, differential signal transmission cables are the predominant choice in the industry.
Differential signal transmission involves the use of a cable having a pair of individual conductors wherein the information or data which is transmitted is represented by a difference in voltage between the individual conductors. The data is represented in transmission by polarity reversals on the conductor pair, and the receiver or other equipment coupled to the receiving end of the cable determines the relative voltage difference between the conductors. The difference is then analyzed to determine its logical value, such as a 0 or 1. Differential pairs may be shielded or unshielded. Shielded differential pairs generally perform better than unshielded pairs because the internal and external environments of the conductors are isolated. Improved attenuation performance also usually results with shielded cables.
Differential signal transmission cables have a variety of desirable electrical characteristics, including immunity to electrical noise or other electrical interferences. Since the differential signals transmitted are generally 180xc2x0 out of phase to provide a balanced signal in the cable, and are considered to be complementary to one another, any noise will affect both of the conductors equally. Therefore, the differences in the signals between the conductors of the pair due to external electrical noise and interference are generally negated, particularly for shielded pairs. It may also be true for unshielded differential pairs as well by varying the twisting of the pairs, for example. It is common to twist the individual conductors of a pair together along the longitudinal axis of the pair. The cables are then referred to as twisted pair cables. The main advantage of such cables is increased mechanical flexibility. However, there are considerable disadvantages to twisted pair cables; two important ones being size increase and high group skew.
Differential signal transmission cables are also generally immune to cross-talk, that is, interference between the individual cables due to the signals on other cables which are bundled together into a multi-cable, or multi-pair, structure. Again, shielded differential pairs will generally outperform unshielded pairs with respect to cross-talk. The multiple differential signal cables bundled together into a larger overall cable structure are referred to as primary cables of the overall, larger cable construction.
Since differential signal transmission relies upon parallel transmission of the data signals through the conductors of a pair, and then comparison of the differences between those signals at the receiving end of the cable, it is desired that the complementary signals of each pair arrive at the receiving end of the cable at the same time. However, properties of the cable affect the propagation speed of the signals along the conductors and therefore introduce delays between the signals of a differential pair. For example, because of insulative property differences experienced by each conductor of a cable pair, such as differences due to dielectric inconsistencies and/or physical characteristics of the cable, differential signal transmission cables are subject to propagation differences between the individual conductors. Variances in the effective length of one conductor with respect to the other conductor of a pair also create such differences. The difference in signal propagation between the conductors of a differential pair and the delays associated therewith is referred to as signal skew. Signal skew is defined as the delay of the arrival of one of the corresponding or complimentary signals at the receiving end with respect to the other signal. In simpler terms, one complimentary signal arrives at the receiving end faster than the other signal, a condition which is exaggerated as cable length increases. Generally, a signal skew budget is designed into data transmission systems and the cables which link the systems are allowed only a portion of the budget.
Within a single differential pair, the skew is determined between the two individual conductors of the pair and is referred to as within-pair skew. In some cable applications, multiple differential pairs are bundled together to form a larger overall cable. Skew is then measured for each pair as a time delay for the differential balanced signal of the cable pair. The measure of time difference between the fastest and slowest signals for each of the multiple pairs, with each pair being considered to provide a single signal, is defined as a pair-to-pair or group skew.
More specifically, with a signal of one conductor considered M1, and the signal of another conductor considered M2, a differential pair will have a propagation delay associated not only with each signal M1, M2 individually, but also with the propagation of the differential balanced signal (M1xe2x88x92M2). The differential balanced signal takes into account the differences in potential along the length of the whole line, the reference limit being zero. As differences in the individual conductors are encountered, each individual conductor of a pair contributes different potentials to the (M1xe2x88x92M2) balanced signal. The (M1xe2x88x92M2) signal fluctuates about zero. The group skew measurement is then the time delay difference between the fastest differential signal (M1xe2x88x92M2) and the slowest of such signals in a group of pairs in a multi-pair cable. That is, (M1xe2x88x92M2) is measured for each pair in a multi-pair cable and then the difference between the maximum time delay and the slowest time delay defines group skew.
Therefore, within-pair and group signal skews are important parameters which must be considered when using a differential signal transmission cable which incorporates multiple differential pairs. As will be appreciated, it is desirable to keep the in-pair signal skew characteristics of a cable to a minimum to prevent errors in communication. Furthermore, low signal skew is necessary for proper cancellation of noise, because if the two opposing signals do not arrive at the receiving end at the same time, a certain amount of the noise in the cable will not be cancelled.
Another important characteristic for a differential signal cable is signal jitter. Signal jitter is defined as the amount of real time it takes for the differential signals"" rising and falling edges to cross over when they transition. Low jitter, or rapid rising and falling edges, is desirable.
Attenuation should also be minimized in a differential cable. All cables will inherently reduce or attenuate the level of the signal transmitted thereon, due to the impedance qualities of the cable. Attenuation is generally affected by the physical structure of the cable, which includes the shield type and design, the dielectric insulation material type, the conductor type, plating type and plating thickness, the position of the conductors with respect to each other, and the electrical interaction between the conductors of the cable. If the primary cables or primaries of a larger multi-paired cable are poorly constructed, the dielectric insulation properties, conductor-to-dielectric geometry, and hence impedance characteristics, may vary along their length. The variation of such impedance characteristics increases the signal attenuation or loss characteristics of the cable. However, attenuation of a test pattern signal, or eye pattern, should be sufficiently low so that suitable triggering voltages will be available at the output of the cable. Accordingly, it is desirable to utilize a cable which has low attenuation characteristics at a desired operating frequency for that cable.
Low within-pair and group skew, low jitter and high signal amplitude (low attenuation) are all desirable characteristics of a differential cable, and improving those characteristics allows a differential signal transmission cable to be utilized at greater lengths or distances. It is therefore desirable to utilize a data transmission cable having a relatively low signal skew, low jitter and low attenuation.
In one aspect of cable design, it is desirable to improve the performance characteristics of a differential cable pair or primary pair. However, multi-pair cables for certain applications use multiple pairs or multiple primary cables which are then bundled together under a common insulative jacket and/or shield. In such a construction, the primaries affect each other, and it is not sufficient to simply design a primary which has desirable characteristics by itself and place it into a bundle with other similar primaries. Rather, the multi-pair cable design must also have the desirable characteristics. That is, a multi-pair cable has its own performance characteristics and criteria which are not dictated solely by the performance of the primaries therein.
Accordingly, it is an objective of the present invention to provide a high-speed data transmission cable which has improved performance characteristics.
It is further desirable to provide such improved performance characteristics in a multi-pair cable.
It is another objective of the invention improve the group skew, within-pair skew, jitter and attenuation characteristics of data transmission cable, and specifically to improve such characteristics for a multi-pair cable.
It is still a further objective of the present invention to provide a high-speed data transmission cable which can be used at greater lengths than the present high speed data cables.
A high speed data transmission cable in accordance with the present invention is comprised of a plurality of primary cables formed together into a larger overall cable structure. Each primary cable includes a pair of generally parallel conductors which are individually insulated, such as with an extruded insulation. The pair of conductors are placed side-by-side and, in one embodiment, an overall layer of insulation simultaneously surrounds the pair of conductors to form the primary cable. In one aspect of the present invention, the overall insulation might be formed by utilizing an unsintered PTFE tape which is wrapped around the pair of conductors. Alternatively, an overall insulation layer may not be used. The primary cable which is formed has opposing short sides and long sides. A shield layer surrounds the overall insulation layer along the length of the cable, to individually electrically isolate the primary cables from each other when they are bundled together. For example, a polyester/metal tape such as PET/aluminum tape, might be wrapped around the primary cable to form the shield. A drain wire might be positioned between the shield layer and the overall insulation layer for grounding the cable.
In accordance with another aspect of the present invention, a plurality of primary cables are positioned around the cable center axis, which may be defined by an elongated plastic insert. A finite number of primary cables are arranged side-by-side with each other and generally parallel with each other to define distinct orbitals around the center axis. The primary cables of the orbitals are positioned to lie generally flat against the center of the cable. That is, a respective long side of each primary cable will generally be facing the center axis. Multiple orbitals are formed around the center axis. In one embodiment of the invention, the primary cables are utilized in a three-orbital construction.
The primary cables of the orbitals, after being arranged in a side-by-side orientation, are wrapped generally helically around the center axis along the length of the cable. Each primary cable remains side-by-side with adjacent primary cables and generally flat around the center axis of the cable, and the conductor pairs of the cable are not significantly individually twisted about each other along the length of the cable. That is, the present invention utilizes flat, primary cables with generally parallel conductors and not twisted conductor pairs.
The overall cable is formed by surrounding the orbitals and the helically wrapped primary cables with an overall shield layer, such as a wrapped polyester/metal tape layer. A metal braid layer is then utilized to surround the overall shield layer. An overall layer of insulation, such as a jacket of insulation, is used to complete the cable.
The unique construction of the present invention has been found to provide desirable within-pair and group skew, jitter, attenuation and other performance characteristics within high speed data transmission applications. These advantages and other advantages will become more readily apparent in the detailed description here and below.